An optical attenuator of the mentioned kind is known from DE-A-3613688. This known attenuator comprises a rotatable filter element which is coated with an attenuation layer the density of which varies with the angle of rotation of the filter element. The attenuation layer usually is a metal coating whose thickness continuously increases along the circumference of the filter element. Different attenuation factors can be adjusted by adjusting different angles of rotation of the filter element. The application of the metal coating on the filter element with the required accuracy involves considerable effort and is time-consuming, thus leading to comparatively high cost of the attenuator. Furthermore, it may come to problems when very high attenuations are desired and when the beam to be attenuated has a very high intensity. In such a case, the attenuation layer has to dissipate a large amount of energy so that it may detach from the filter element.
Another problem encountered with optical attenuators of the mentioned type is that the attenuation is dependent on the polarization of the light beam. Such polarization dependence is caused by anisotropies in the attenuation layer which may be due to the manufacturing process wherein metal is deposited on a substrate and wherein deposited metal particles may be oriented in preferred directions. This effect is particularly disturbing in fiber optics wherein the polarization of the light can already be changed by bending or twisting the fiber which would cause an unwanted change in the attenuation.